The invention relates to a method for measuring the rotation speed of an induction machine whose stator is connected via a controllable AC controller to a single-phase or polyphase AC power supply system. The invention also relates to a device for determining the rotation speed of an induction machine.
It is known for controllable AC controllers to be used for matching the electric volt-amperes supplied to an induction machine to the respectively prevailing load conditions, in particular during starting and braking.
Such a microprocessor-controlled AC controller or soft starter, as is known for example from EP 0 454 697 B1, operates using the phase-gating principle and is used essentially for smooth starting and stopping of three-phase asynchronous machines. Three sets of active devices, in general each including two back-to-back connected thyristors, are generally actuated by a microprocessor for this purpose.
The control device in the known three-phase controller has no information about the present rotation speed of the machine. With certain mechanical load conditions, this can lead to poor operation of the overall drive. When stopping a pump drive, an abrupt drop in rotation speed can occur, which can lead to extremely high pressures in the pipeline system and thus to severe mechanical loads, and even to destruction of the system. A corresponding situation applies to the starting of drives when a sudden rise in the rotation speed occurs.
If the rotation speed is known, the control device, generally a microprocessor, can be used to provide rotation speed control which allows largely smooth starting and stopping of the drive, even when the mechanical load conditions are poor.
DE 27 15 935 A1 discloses a starting monitor for asynchronous machines, in which the phase angle between the current and voltage is determined. This is used to derive binary information about the starting of the machine. If starting does not take place within a specific time period, the machine is disconnected from the power system once again in order to avoid thermal overloads.
In U.S. Pat. No. 5,548,197A, the current zero crossings of the three stator currents are detected using, inter alia, the voltage which can be measured across the thyristors for this purpose. Two immediately successive current zero crossings are used to form an error signal by subtracting the times of the zero crossings from one another and then subtracting one-sixth of the power supply system period. The error signal, which fluctuates about the zero point, is subjected to frequency analysis, and the rotation speed of the rotor is determined from this. Power supply system disturbances can in this case result in corruption of the measurement signal.
A method which measures the polarity of the induced terminal voltage during the process of stopping an induction machine by means of a three-phase controller and which determines the rotation speed from the time difference between the polarity changes of the individual voltages is described in EP 0 512 372 B1. In any case, during the stopping process, there are time periods in which the induction machine is disconnected from the power supply system and in which, in consequence, no currents flow in the stator either. There is thus no need to interrupt the current supply solely to measure the rotation speed. In this case, in order to brake the induction machine, specific trigger sequences of thyristors are defined in advance, and the time offset of the respective polarity change is evaluated as the frequency for determining the rotation speed. On the other hand, no method is specified for general starting and stopping.
U.S. Pat. No. 5,644,205A and DE 195 03 658 C3 each indicate a method for measuring the rotor angular velocity for machines using frequency-changing control. These methods use the frequency of the induced voltage once the power supply has been disconnected from the machine to determine the rotor angular velocity. Owing to the considerably different functional principles of frequency changers and three-phase controllers, the method of producing a stator without current, which is known from the cited documents, cannot be transferred to machines controlled by three-phase controllers.
Against the background of the prior art, the invention is now based on the object of specifying a method for measuring the rotation speed of an induction machine, which can be carried out easily during acceleration during the starting of the induction machine and in which there is no need for any additional measured value sensors for detecting the rotation speed. Furthermore, the invention is based on the object of specifying a device for controlling such an induction machine.
According to the invention, the first-mentioned object is achieved by a method for measuring rotation speed of an induction machine whose stator is connected, via a controllable AC controller having active device arrangements, to an AC power supply. The method includes controlling the active device arrangements to disconnect the stator from the AC power supply system for at least one predetermined time period (xcex94t), which is less than half of a period (T) of a voltage of the AC power supply system, by controlling the active device arrangements; measuring in the time period (xcex94t), a voltage which is induced in the stator by rotary movement of a rotor and using the measured voltage to determine components of a stator voltage space vector; and determining a rotation frequency of the stator voltage space vector from the measured voltage, and deriving the rotation speed of the induction machine therefrom. In the method for measuring the rotation speed of an induction machine whose stator is connected via a controllable AC controller to a single-phase or polyphase AC power supply system, the stator is disconnected from the AC power supply system for at least a predetermined time period by controlling the active devices in the AC controller. At least one stator voltage, which is induced in the stator by the rotary movement of the rotor, is measured in this time period. The measured values obtained in this way are used to determine the frequency of this stator voltage, and the rotation speed of the induction machine is derived from this.
The stator is thus temporarily placed in a situation where no current is flowing during acceleration of the induction machine. During the time period in which no stator current is flowing, a slowly decaying direct current flows in the rotor. As a result of this, the rotor can be regarded as a rotating magnet with virtually constant magnetic flux, with respect to the rotor coordinate system. The rotation induces voltages (terminal voltages) across the stator terminals of the induction machine, whose frequency corresponds to the product of the known number of pole pairs p and the mechanical rotation speed to be measured.
According to the invention, the rotation speed of the rotor is detected using the frequency of the stator voltage space vector, which can be determined on the basis of the induced voltage, during a time period which is produced deliberately with the aid of the controllable AC controller and in which no current flows in the stator. According to the invention, the time duration of this time period is shorter than the time duration of half the period of the power supply system voltage, in order to influence the operation of the drive only to a minor extent.
For the same reasons, in a further preferred refinement of the invention, the rotation speed measurement in accordance with the abovementioned method is repeated after specific time periods, which are preferably 5 to 15 times the period of the power supply system voltage.
Thyristors are preferably used as the active device arrangements, and the induction machine is disconnected from the AC power supply system by omitting the trigger signals required to trigger the thyristors.
In one particularly preferred refinement of the invention, in order to resume the power supply system operation of the induction machine in the case of a three-phase induction machine, the first trigger signal for the first-opened first active device arrangement in one phase is delayed by a multiple of half the power supply system period with respect to the last trigger signal for this first active device arrangement. At the same time as the renewed triggering of this first active device arrangement, a second active device arrangement is triggered, which is an active device arrangement that is triggered subsequently in normal operation. The third active device arrangement is triggered one-sixth of the power supply system period after the triggering of the first active device arrangement, with the trigger signal sequence which was present before the disconnection then being reproduced. This measure ensures that the interruption in the voltage supply to the induction machine which follows the rotation speed measurement has as little influence as possible on the continued operation of the induction machine.
In a further advantageous refinement of the method, during the time period during which no current is flowing in the stator in the case of a polyphase AC power supply system, the terminal voltages which are in each case induced in the stator windings between the stator terminals are measured. The angle of the space vector of the induced stator voltage is in each case calculated, in particular, from the measured values of the terminal voltage.
For discrete-time sampling of the induced terminal voltage, the clock rate is in this case defined such that the associated angles of the space vector of the induced stator voltage are calculated for as many times as possible within the time period. The determined angles of the space vector are associated, within the time period during which no current is flowing in the stator, with a straight line from whose gradient the rotation speed of the induction machine is determined.
According to the invention, the second-mentioned object is achieved by a device for determining rotation speed of an induction machine whose stator is connected via an AC controller to an AC power supply system. The device includes a control device for controlling the AC controller, and for disconnecting the stator from the AC power supply system for a predetermined time period (xcex94t), which is shorter than half a period (T) of a voltage of the AC power supply system, by controlling active device arrangements of the AC controller; a voltage measurement device for measuring at least one terminal voltage which is induced in the stator by rotary movement of a rotor in the time period (xcex94t); and a computation device for calculating a frequency of the measured terminal voltage and for calculating the rotation speed of the induction machine from the calculated frequency, wherein a control signal for the control device is present at one output of the computation device, the control signal being derived from the rotation speed and being passed to the control device. The device for controlling an induction machine, whose stator is connected via an AC controller to a single-phase or polyphase AC power supply system, contains a control device for controlling the AC controller and for disconnecting the stator from the AC power supply system for a predetermined time period by opening the active device arrangements in the AC controller. It further includes a voltage measurement device for measuring at least one stator voltage which is induced in the stator by the rotary movement of the rotor in this time period. Finally, a computation device is included, for calculating the frequency of this stator voltage from the measured values obtained in this way, and for calculating the rotation speed of the induction machine from this frequency.
In one preferred embodiment, the rotation speed is used to derive a control signal for the control device. This control signal is produced at one output of the computation device and is passed via a control line to the control device.
Further preferred embodiments of the device are evident from the subsequent description of the present invention.